of 21
Chapter
14
Magmas:
windows
into
the
mantle
If
our
eye
could
penetrate
the
Earth
and
see
its
interior
from
pole
to
pole,
from
where
we
stand
to
the
antipodes,
we
would
glimpse
with
horror
a mass
terrifying
riddled
with
fissures
and
caverns
Tel/us
Theoria
Sacra
(
1694),
Thomas
Burnet
Various
ldnds
of
magmas,
ranging
from
mido-
cean-ridge
basalts
(MORE)
to
ldmberlite
(KIMB)
emerge
from
the
mantle
.
Material
is
also
recycled
into
the
mantle;
sediments,
oceanic
crust,
delam-
inated
continental
crust,
water
and
peridotite
.
Parts
of
the
mantle
are
inaccessible
to
direct
sampling
and
we
can
only
infer
their
composi-
tions
by
subtracting
off
the
sampled
components
from
what
is
thought
to
be
the
original
composi-
tion.
There
is
no
assurance
that
we
are
currently
receiving
samples
from
all
parts
of
the
Earth
,
although
this
is
the
hope
of
many
geochemists.
There
is
reason
to
believe
that
the
chem.ical
strat-
ification
of
the
mantle
is
irreversible,
and
that
there
exist
hidden
'reservoirs'
(a
bad
term
for
a
permanent
repository)
.
Nevertheless,
the
accre-
tional
stratification
of
the
Earth
may
have
placed
most
of
the
low-melting
point
lithophile
ele-
ments
within
reach.
In
fact,
one
can
construct
a
plausible
compositional
model
for
the
mantle
by
isolating
the
dense
refractory
depleted
products
of
differentiation
in
the
deep
mantle.
This
would
make
the
Mg
.
Si
and
Fe
contents
of
the
mantle
uncertain
but
these
can
be
constrained
by
geo-
physics,
mineral
physics
and
cosmology
. In
fact,
if
one
mixes
together
all
the
materials
known
to
enter
or
leave
the
mantle,
one
can
obtain
cosmic
ratios
of
the
refractory
elements
except
for
Mg/Si
and
other
elements
likely
to
enter
dense
refrac-
tory
residues
of
mantle
differentiation
. Likewise.
this
mix
is
deficient
in
the
siderophile
elements,
which
are
plausibly
in
the
core,
and
the
volatile
elements,
which
were
probably
excluded
from
the
beginning.
Magmas
are
an
important
source
of
infor-
mation
about
conditions
and
composition
of
the
Earth's
interior
.
The
bulk
composition.
trace-
element
chemistry,
isotope
geochemistry
and
volatile
content
of
magmas
all
contain
infor-
mation
about
the
source
region
and
the
pro-
cesses
that
have
affected
the
magmas
before
their
eruption.
Mantle
fragments,
or
xenoliths
,
found
in
these
magmas
tell
us
about
the
mate-
rial
through
which
the
magmas
have
passed
on
the
way
to
the
surface.
Representative
composi-
tions
of
various
magmas
are
given
in
Table
14
.1.
Three
principal
magma
series
are
recognized
:
tholeiite,
calc-alkaline
and
alkali
.
TI1e
various
rock
types
in
each
series
may
be
related
by
vary-
ing
degrees
of
partial
melting
or
crystal
sepa-
ration.
The
dominant
rock
type
is
tholeiite,
a
fine-grained
dark
basalt
containing
little
or
no
olivine.
Tholeiites
are
found
in
both
oceanic
and
intraplate
settings.
Those
formed
at
mido-
cean
ridges
are
low-potassium
and
LIL-depleted
and
relatively
high
in
AI,
while
those
found
on
oceanic
islands
and
continents
are
generally
LIL-
enriched.
Ridge
tholeiites
differ
from
continental
and
island
tholeiites
by
their
higher
contents
of
AI
and
Cr.
low
contents
of
large-ion
lithophile
MAGMAS:
WINDOWS
INTO
THE
MANTLE
1
69
Table
14.1
I
Representative
compositions
of
basalts
and
andesites.
Tholeiite
Oceanic
Cont.
Rift
Continental
Alkali
Alkali
Andesite
Ridge
Arc
Rift
Island
High-
AI
Basalt
Basalt
Arc
Low-K
High-K
Si0
2
49.8
51.1
50.3
49.4
51.7
47.4
47.8
57.3
59.5
60.8
Ti0
2
1.5
0.83
2.2
2.5
1.0
2.9
2
.2
0.58
0.70
0.77
AI
20 3
16.0
1
6.1
14.3
13.9
16.9
18.0
153
17.4
17.2
16.8
Fe
20 3*
10.0
I
1.8
13.5
12.4
11.6
10
.6
12.4
8.1
6.8
5.7
MgO
7.5
5.1
5.9
8.4
6.5
4.8
7.0
3.5
3.4
2.2
CaO
11.2
10.8
9.7
10.3
11.0
8.7
9.0
8.7
7.0
5.6
Na
2
0
2.75
1.96
2.50
2.13
3.10
3.99
2.85
2.63
3.68
4.10
K20
0.14
0.40
0.66
0.38
0.40
1.
66
1.
31
0.70
1.6
3.25
Cr
300
50
160
250
40
67
400
44
56
3
Ni
100
25
85
ISO
25
50
100
15
18
3
Co
32
20
38
30
50
25
60
20
24
13
Rb
I
5
31
5
10
33
200
10
30
90
Cs
O.Q2
0.05
0.2
0.1
0.3
2
>3
~0.1
0.7
1.5
Sr
135
225
350
350
330
800
1500
2 15
385
620
Ba
II
50
170
100
115
500
700
100
270
400
Zr
85
60
200
125
100
330
800
90
110
1
70
La
3.9
3.3
33
7.2
10
17
54
3.0
12
13
Ce
12
6.7
98
26
19
50
95
7.0
24
23
Sm
3.9
2.2
8.2
4.6
4.0
5.5
9.7
2.6
2.9
4.5
Eu
1
.4
0.76
23
1.6
1.3
1.9
3.0
1.0
1.0
1.4
Gd
5.8
4.0
8.1
5.0
4.0
6.0
8.2
4.0
3.3
4.9
Tb
1
.2
0.40
1.1
0.82
0.80
0.8
1
2.3
1.0
0.68
1.1
Yb
4.0
1.9
4.4
1.7
2.7
1.5
1.
7
2.7
1.9
3.2
u
0.10
0
.1
5
0.4
0.18
0.2
0.75
0.5
0.4
0.7
2.2
Th
0
.1
8
0.5
1.5
0.67
1.1
4.5
4.0
1.3
2.2
5.5
Th/U
1.8
3.3
3.8
3.7
5.9
6.0
8.0
3.2
3.1
2.5
K/Ba
10
5
66
32
32
12
28
16
58
49
68
K/Rb
1160
660
176
630
344
420
55
580
440
300
Rb/Sr
0.007
0.022
0.089
0.014
0.029
O.Q45
0.
13
0.046
0.078
0.145
La/Yb
1.0
1.7
10
4.2
3.7
II
32
1.1
6.3
4.0
*Tota
l
Fe
as
Fe
2
0
3
.
(After
Candie,
1982
.)
(LIL)
elements
(such
as
K, Rb,
Cs,
Sr,
Ba,
Zr,
U,
The
world-encircling
midocean-ridge
system
Th
and
REE)
and
depleted
isotopic
ratios
(i.e.
accounts
for
more
than
90%
of
the
material
flow-
low
integrated
Rb
/S
r,
U/Pb
etc.
ratios).
Tholeiitic
ing
out
of
the
Earth's
interior.
The
whole
ocean
basalts
have
low
viscosity
and
flow
for
long
dis-
floor
has
been
renewed
by
this
activity
in
less
tances,
constructing
volcanic
forms
of
large
area
than
200
Ma.
Hotspots
represent
less
than
10%
of
and
small
slope.
The
major-
and
trace-element
dif-
the
material
and
the
heat
flow.
The
presence
of
a
ferences
between
the
tholeiitic
and
calc-alkaline
crust
and
core
indicates
that
the
Earth
is
a differ-
suites
can
be
explained
by
varying
proportions
entiated
body
. Major
differentiation
was
contem-
of
olivine,
plagioclase
and
pyroxene
crystallizing
poraneous
with
the
accretion
of
the
Earth
.
The
from
a
basaltic
parent
melt.
high
concentrations
of
incompatible
elements
in
170
MAGMAS:
WINDOWS
INTO
THE
MANTLE
the
crust
and
the
high
argon-40
content
of
the
atmosphere
indicate
that
the
differentiation
and
degassing
has
been
relatively
efficient.
The
pres-
ence
of
helium-3
in
mantle
magmas
shows,
how-
ever,
that
outgassing
has
not
been
100%
effi-
cient.
The
evidence
fi·om
helium
and
argon
is
not
contradictory.
Helium
dissolves
more
read-
ily
in
magma
than
argon
and
the
heavy
noble
gases
so
we
expect
helium
degassing
to
be
less
effective
than
argon
degassing.
We
also
have
no
constraint
on
the
initial
helium
content
of
the
mantle
or
the
total
amount
that
has
degassed.
So
helium
may
be
mostly
outgassed
and
what
we
see
is
just
a
small
fraction
of
what
there
was.
TI1e
evidence
for
differentiation
and
a
hot
early
Earth
suggest
that
much
of
the
current
magmatism
is
a
result
of
recycling
or
the
processing
of
already
processed
material.
TI1e
presence
of
helium-3
in
the
mantle
suggests
that
a
fr-action
of
the
magma
generated
remains
in
the
mantle;
that
is,
magma
removal
is
inefficient.
Generalities
In
most
models
of
basalt
genesis,
it
is
assumed
not
only
that
olivine
and
orthopyroxene
are
con-
tained
in
the
source
rock
but
that
these
are
the
dominant
phases.
Petrologic
and
isotope
data
alone,
however,
cannot
rule
out
a
source
that
is
mainly
garnet
and
clinopyroxene.
The
eclogite
(garnet
plus
clinopyroxene)
source
hypothesis
dif-
fers
only
in
scale
and
melt
extraction
mechanism
from
the
fertile
peridotite
hypothesis.
In
the
fer-
tile
peridotite,
or
pyrolite,
model
the
early
melt-
ing
components,
garnet
and
clinopyroxene,
are
distributed
as
grains
in
a
predominantly
olivine-
orthopyroxene
rock.
On
a
larger
scale,
eclogite
might
exist
as
pods
or
blobs
in
a
peridotite
rnan-
tle.
Since
eclogite
is
denser
than
peridotite,
at
least
in
the
shallowest
mantle,
these
blobs
would
tend
to
sink
and
coalesce.
In
the
extrerne
case
an
isolated
eclogite-rich
layer
might
form
below
the
lighter
peridotite
layer
.
Such
a
layer
could
form
by
crustal
delamination,
subduction
or
by
crystal
settling
in
an
ancient
magma
ocean.
Melts
from
such
blobs
or
layers
still
interact
with
olivine
and
orthopyroxene.
If
eclogite
blobs
are
surrounded
by
refractory
peridotite
they
can
extensively
melt
upon
adiabatic
decompression,
without
the
melt
draining
out.
The
isotopic
evidence
for
isolated
reservoirs
and
the
geophysical
evidence
for
gross
layering
suggest
that
differentiation
and
chemi-
cal
stratification
may
be
more
important
in
the
long
run
than
mixing
and
homogenization.
Fluids
and
small-degree
melts
are
LIL-
enriched,
and
they
tend
to
migrate
upward.
Sedi-
ments
and
altered
ocean
crust,
also
LIL-enriched,
re-enter
the
upper
mantle
at
subduction
zones.
TI1us
there
are
several
reasons
to
believe
that
the
shallow
mantle
serves
as
a
scavenger
of
incompat-
ible
elements,
including
the
radioactive
elements
(U,
Th
and
K)
and
the
key
tracers
(Rb,
Sr,
Nd,
Sm
and,
possibly,
Pb
and
C0
2
).
The
continental
crust
and
lithosphere
are
commonly
assumed
to
be
the
main
repositories
of
the
incompatible
elements,
but
oceanic
islands,
island
arcs
and
deep-seated
kimberlites
also
bring
LIL-enriched
material
to
the
s
urface
. Even
a
moderate
amount
of
LIL
in
the
upper
mantle
destroys
the
arguments
for
a
primitive
lower
mantle
or
the
need
for
a
deep
radioactive-rich
layer.
It
is
becoming
increasingly
clear
that
all
magma
are
blends
of
melts
from
an
inhomo
ge-
nous
mantle.
In
fact,
the
source
of
magma
has
been
described
as
a
statistical
upper
man-
tle
assemblage,
or
SUMA.
Various
apparent
inconsistencies
between
the
trace
element
ratios
and
isotopic
ratios
in
basalt
can
be
understood
if
(1)
partial
melting
processes
are
not
100%
efficient
in
removing
volatiles
and
incompati-
ble
e
lements
fi·om
the
mantle;
(2)
basalts
are
hybrids
or
blends
of
magmas
from
depleted
and
enriched
components;
and
(3)
different
basalts
represent
different
averages,
or
different
volume
sampling.
In
a
chemically
heterogenous
man-
tle
mixing
or
contamination
is
inevitable.
Even
with
this
mixing
it
is
clear
fi·om
the
isotopes
that
there
are
four
or
five
ancient
components
in
the
mantle,
components
that
have
had
an
independent
history
for
much
of
the
age
of
the
Earth.
Some
components
in
a
heterogenous
man-
tle
melt
before
others.
Material
rising
from
one
depth
level
advects
high
temperatures
to
shallow
levels,
and
can
cause
melting
from
the
material
in
the
shallow
mantle.
It
is
not
necessary
that
material
melt
itself
in
situ.
Cold
but
fertile
sub-
ducted
or
delaminated
material
will
melt
as
it
warms
up
to
ambient
mantle
temperature;
excess
temperatures
are
not
required.
Thus,
there
are
a
variety
of
ways
of
generating
melting
anomalies.
In
a
stratified
mantle,
without
transfer
of
material
between
layers,
convection
in
a
lower
layer
can
control
the
location
of
melting
in
the
overlying
layer.
In
a
homogenous
mantle,
the
high
temperature
gradient
in
the
thermal
bound-
ary
layer
between
layers
is
the
preferred
source
of
magma
genesis
since
the
melting
gradient
is
larger
than
the
adiabatic
gradient
in
a
homoge-
nous
convecting
fluid
(Figure
14.5).
In
a
homoge-
nous
mantle,
if
the
melting
point
is
not
exceeded
in
the
shallow
mantle,
then
it
is
unlikely
to
be
exceeded
at
greater
depth
since
the
me
l
t-
ing
point
and
the
geotherm
diverge
with
depth.
Material
can
leave
a
deep
source
region
by
several
mechanisms.
(1)
Melting
in
the
thermal
boundary
at
the
bot-
tom
or
the
top
of
a
region
because
of
the
high
therma
l
gradient.
(2)
Melting,
or
phase
changes,
due
to
adiabatic
ascent
of
hotter
or
lower
melting
point
regions.
(3)
Entrainment
of
material
by
adjacent
convect-
ing
layers.
(4)
Heating
of
fertile
blobs
by
internal
rad
ioac-
tivity
or
by
conduction
of
heat
from
the
sur-
rounding
mantle
.
Some
of
these
mechanisms
are
illustrated
in
Fig-
ure
7.1.
Midocean
ridge
basalts
TI1e
most
voluminous
magma
type,
MORB,
has
low
LIL-content
and
depleted
isotopic
r
atios,
with
very
low
variance,
that
is,
they
are
homogeneous.
The
Rb/Sr,
Nd/Sm
and
U/Pb
ratios
have
been
low
since
>
1
Ga.
Since
these
ratios
are
high
in
melts
and
low
in
residual
crystals,
the
implication
is
that
the
MORB
source
is
a
cumulate
or
a
crys-
talline
residue
remaining
after
the
removal
of
a
melt
fraction
or
residual
fluid.
Extraction
of
very
enriched,
very
small-degree
melts
(
< 1
%)
is
implied.
In
some
geochemical
models
the
LIL-
enriched
melt
fraction
is
assumed
to
efficiently
leave
the
upper
mantle
(the
depleted
MORE-
source
in
these
models)
and
enter,
and
become
,
the
continental
crust.
TI1e
continental
crust
is
therefore
regarded
as
the
complement
-
often
MAGMAS
:
WINDOWS
INTO
THE
MANTLE
171
the
only
comp
l
ement
-
to
the
depleted
MORB
source.
Early
mass-balance
calculations
based
on
this
premise,
and
on
just
a
few
elements
and
isotopes
,
suggested
that
most
of
the
mantle
remains
undepleted
or
primitive
.
The
depleted
reservoir
was
assumed
to
occupy
most
or
all
of
the
upper
mantle.
Since
the
continental
crust
is
the
on
ly
enriched
reservoir
in
this
three-reservoir
model,
magmas
that
have
high
LIL-contents
and
87
Srf
86
Sr
ratios
are
assumed
to
be
contaminated
by
t h e
continental
crust,
or
to
contain
a recycled
anc
i
ent
crusta
l
component.
A
depleted
reservoir
(low
in
LIL
,
low
in
Rb
/Sr,
87
Srf
86
Sr
and
so
on)
can
still
be
fertile,
i.e.
it
can
provide
basalts
by
partia
l
melting.
A
clinopyrox-
enite
or
gabbro
cumulate,
for
example,
can
be
depleted
but
fertile.
Similarly,
an
enriched
reser-
vo
ir
can
be
infertile
,
being
low
in
Ca,
Al,
Na
and
so
on.
The
fact
that
most
of
the
mantle
must
be
depleted
as
implied
by
mass-balance
calcula-
tions
,
does
not
mean
that
it
is
fertile
or
similar
to
the
MORB
reservoir.
Most
of
the
volume
of
the
mantle
is
depleted
infertile-
or
barren-refractory
residue
of
terrestrial
accretion,
and
most
of
it
is
magnesium
perovskite
in
the
lower
mantle.
Ocean-island
basalts
The
trace
element
and
isotope
ratios
of
basalts
from
ocean
islands
(OIB)
differ
from
otherwise
sim
il
ar
MORB.
There
is
still
no
general
agreement
on
how
such
variations
are
produced;
chemical
and
isotope
variability
of
the
mantle
is
likely
to
be
present
everywhere,
not
just
in
the
source
regions
of
OIB.
Both
geochem
ical
and
geophys-
ical
observations,
and
plate-tectonic
processes,
require
the
upper
mantle
to
be
inhomogenous,
and
a
variety
of
mechanisms
have
been
suggested
to
produce
such
inhomogeneities.
Perhaps
the
most
obvious
model
for
generating
a
variably
fer-
tile
and
inhomogeneous
mantle
is
subduction,
delamination
and
incomplete
melt
extraction.
Variability
is
ge
nerated
by
recyclin
g
of
oceanic
and
continental
crust,
and
of
seamounts,
aseis-
mic
ridges,
oceanic
islands
and
sediments,
all
of
which
are
bein
g
transported
into
subduction
zones.
Recycling
of
the
oceanic
crust
involves
the
grea
test
mass
flux
of
these
various
compo-
nents
but
this
does
not
require
that
such
mate-
rial
is
the
dominant
so
ur
ce
of
OIB.
A
popular